In general, heat generation of a solenoid valve is unfavorable. Thus, much effort has been spent for heat generation saving and electric power saving of solenoid valves so far. For example, a conventional holding circuit 51 shown in FIG. 7 reduces rated voltage to holding voltage by connecting a resistance 59 to a solenoid 53 in series. FIG. 8 illustrates the temporal change of voltage at the solenoid 53 in FIG. 7. FIG. 9 illustrates the relation between applied voltage and magnetic flux density at the solenoid 53.